The present invention relates to a power transmission for use on a vehicle equipped with four wheel drive and, more particularly, to a power transmission of the type having a transfer gear and a center-differential gear for driving front and rear wheel driving shafts which are laterally spaced apart and located in parallel with each other.
Power transmissions used on vehicles equipped with four wheel drive include a transfer gear assembly shiftable between different speed drive ranges, a center-differential gear assembly shiftable between different drive modes, and shift mechanisms, one for each gear assembly for shifting the gear assemblies between their different operating positions. As is well known to those skilled in the art, the transfer gear assembly is shifted between a high speed and a low speed drive range, and the center-differential gear assembly is shifted between two wheel drive mode (which is hereinafter noted by 2WD on descriptive listings) and four wheel drive mode (which is hereinafter noted by 4WD on descriptive listings) and between differential-free drive mode (which is hereinafter noted by Diff-Free on descriptive listings) wherein front and rear wheel drive shafts are driven at the same time but allowed to turn at different speeds and differential-locked drive mode (which is hereinafter noted by Diff-Locked) wherein the front and rear wheel drive shafts are driven at the same time and at the same speed.
One disadvantage associated with these known power transmissions is that they tend to be quite complex in construction and long in axial length, thereby making the power transmissions unfavorable and difficult to be installed on vehicles. In an attempt at overcoming such a disadvantage of this type of power transmission, some transfer and centerdifferential gear assemblies are designed to utilize advantageous constructional feature of planetary gear units to devise a power transmission which is simpler in construction and shorter in overall axial length. One such power transmission is disclosed in, for example, Japanese Patent Unexamined Publication No. 61-223,366, entitled "Power Transmission for a Four Wheel Drive Vehicle," laid open Oct. 3, 1986. This power transmission is, however, still relatively complex in its mechanisms because the planetary gear units used as the transfer and the center-differential gear assemblies are partly shared by the two gears. Specifically, shifting the power transmission between different drive modes has to be effected by three shift mechanisms individually operated. Due to the provision of three shift mechanisms, the power transmission is still complex in construction and long in overall axial length.
In addition to the complexity of the power transmission, there is an operational problem associated with the power transmission. The shift mechanism for the center-differential gear assembly of a conventional power transmission generally consists of three clutch ring members axially spaced and located coaxially relative to the planetary gear unit and an axially movable clutch sleeve selectively engageable with the three clutch ring members.
Where the shift mechanism is to cooperate with the center-differential gear assembly of the power transmission, there is a design requirement that each clutch gear ring member has to be given a certain rotational play either on a drive shaft or on a sleeve on which the clutch ring members are mounted for rotation. This particular requirement is necessitated by a feature of this shifting mechanism, namely, that relative rotational phases of the respective clutch ring members change uncertainly and independently during rotation. If each clutch ring member is given no rotational play, it is quite difficult to bring smoothly the clutch sleeve into and out of engagement with any one of the three clutch ring members during rotation, resulting in a troublesome drive mode shift operation of the power transmission.